Constraining Cosmic-ray Transport with Observations of the Circumgalactic Medium
| Autor: | Iryna S Butsky, Shreya Nakum, Sam B Ponnada, Cameron B Hummels, Suoqing Ji, Philip F Hopkins |
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| Rok vydání: | 2022 |
| Předmět: | |
| DOI: | 10.48550/arxiv.2210.14232 |
| Popis: | Recent theoretical studies predict that the circumgalactic medium (CGM) around low-redshift, $\sim L_*$ galaxies could have substantial nonthermal pressure support in the form of cosmic rays. However, these predictions are sensitive to the specific model of cosmic-ray transport employed, which is theoretically and observationally underconstrained. In this work, we propose a novel observational constraint for calculating the lower limit of the radially-averaged, effective cosmic-ray transport rate, $\kappa_{\rm min}^{\rm eff}$. Under a wide range of assumptions (so long as cosmic rays do not lose a significant fraction of their energy in the galactic disk, regardless of whether the cosmic-ray pressure is important or not in the CGM), we demonstrate a well-defined relationship between $\kappa_{\rm min}^{\rm eff}$ and three observable galaxy properties: the total hydrogen column density, the average star formation rate, and the gas circular velocity. We use a suite of FIRE-2 galaxy simulations with a variety of cosmic-ray transport physics to demonstrate that our analytic model of $\kappa_{\rm min}^{\rm eff}$ is a robust lower limit of the true cosmic-ray transport rate. We then apply our new model to calculate $\kappa_{\rm min}^{\rm eff}$ for galaxies in the COS-Halos sample, and confirm this already reveals strong evidence for an effective transport rate which rises rapidly away from the interstellar medium to values $\kappa_{\rm min}^{\rm eff}\gtrsim 10^{30-31}\,{\rm cm}^2\,{\rm s}^{-1}$ (corresponding to anisotropic streaming velocities of $v^{\rm stream}_{\rm eff} \gtrsim 1000\,{\rm km}\,{\rm s}^{-1}$) in the diffuse CGM, at impact parameters larger than $50-100$\,kpc. We discuss how future observations can provide qualitatively new constraints in our understanding of cosmic rays in the CGM and intergalactic medium. Comment: 9 pages, 2 figures, accepted to MNRAS |
| Databáze: | OpenAIRE |
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